Photometry of Proxima Centauri and Barnard's Star Using HST Fine Guidance Sensor 3: A Search for Periodic Variations

摘要

We have observed Proxima Centauri and Barnard's Star with Hubble SpaceTelescope Fine Guidance Sensor 3. Proxima Centauri exhibits small-amplitude,periodic photometric variations. Once several sources of systematic photometricerror are corrected, we obtain 2 milli-magnitude internal photometricprecision. We identify two distinct behavior modes over the past four years:higher amplitude, longer period; smaller amplitude, shorter period. Within theerrors one period (P ~ 83d) is twice the other. Barnard's Star shows very weakevidence for periodicity on a timescale of approximately 130 days. If weinterpret these periodic phenomena as rotational modulation of star spots, weidentify three discrete spots on Proxima Cen and possibly one spot on Barnard'sStar. We find that the disturbances change significantly on time scales asshort as one rotation period.